370 



Messrs. du Boia and Rubens on Refraction 



tion according as a or n be considered given in addition to 

 i and /3. A discussion of equation (4) shows that at the 

 minimum of deviation (for ^ = 0) 



*^j3(n-l) [or n= |+1]. 



(5) 



Of course # changes but slightly near its minimum (compare 

 A. p. 259). When i approaches 90°, # tends towards infinite 

 values ; at least so long as n > I, and accordingly no total 

 reflexion ensues (see § 15). 



5 11. The experiments were begun by adjusting the biprism 

 vertically on the spectrometer platform. By means of the 

 latter's divided circle, the normal to the prism could be set 



Table I. (Prism Fe III,). 



0. 



Deviation « at Inclination i : 



0°. 



±30°. 



±40°. 



±50°. 



±55°. 



±60°. 



±65°. 



du Bois. 



26-1 

 25 

 25-5 



51-4 

 50-7 

 50-4 

 52-0 



63-7 

 67-5 



74-7 



72-2 



91-6 



107-2 



1053 

 105-7 



^22-5 

 122^8 



147-0 

 159-5 



257 



51-1 



65-6 



73 5 



99-4 



105-5 



122-7 



153-3 



Rubens. 



27-8 

 23-4 

 264 



51-5 



51-2 

 531 

 53-8 



655 



70-2 



74-9 

 71-4 



88-5 

 106-8 



1144 



103-7 



119-5 

 1379 



1555 

 153-5 



25-9 



52-4 



67-9 



732 



97-7 



109-1 



128-7 



154-5 



Mean Values. 



25-8 



51-7 



66-7 



73-3 



98-5 



107-3 



125-7 



153-9 



